An acceleration sensor including a weight layer and a piezoelectric ceramic element which is polarized in a predetermined direction is known. In such an acceleration sensor, when inertial force acts on the weight layer due to external acceleration, electric charge is generated in the piezoelectric ceramic element due to the inertial force, and it is possible to detect vibration and to detect the magnitude of acceleration based on the electric charge.
The acceleration sensor can be electrically and mechanically connected to a circuit substrate by soldering. In this case, a solder fillet formed integrally with a terminal electrode of the acceleration sensor and the circuit substrate may limit movement, particularly in a slide direction (a direction parallel to a substrate surface). When the movement in the slide direction is limited in this manner, it may be difficult for the generated electric charge to correspond to a load caused by the acceleration accurately and it is difficult to obtain high detection accuracy.
Therefore, in Japanese Patent Application Publication No. 2007-101448, a piezoelectric layer in which electric charge is generated is disposed away from a circuit substrate so that movement of the piezoelectric layer is not easily limited by a solder fillet. In this way, the electric charge corresponds to a load caused by acceleration more accurately.
In Japanese Patent Application Publication No. 2007-101448, the piezoelectric layer has an inwardly depressed shape to realize a structure in which a solder fillet is not easily formed in the inwardly depressed portion and the operation of an acceleration sensor is less easily limited by the solder fillet.
As a result of intensive studies, the present inventors have found a new technique for improving the sensitivity of an acceleration sensor by making the operation of the acceleration sensor less easily limited by a solder fillet.